Machining of steels not only affects the shape of the workpiece, but also influences surface structure and composition. At elevated temperatures and pressures during machining or the subsequent cleaning process cooling lubricants, hydraulic oils, machine greases, and cleaning residues can form stable adsorption or reaction layers. These layers can work as barriers and therefore inhibit surface modification reactions, as can be seen in the example of gas nitriding. The aim of the present investigation was to explore possible mechanisms of reaction inhibition and develop a nitriding process more reproducible in terms of reaction temperature and residence time. Samples prepared with residues of cleaning and machining processes on different alloy surfaces were nitrided under normal conditions and with extremely short residence times (1 < t < 20 min), in order to detect the effect of reaction and barrier layers on nitrogen acceptance during the initial stages of the nitriding process. For all alloys examined the samples with adsorbed cleaning agents showed the worst nitriding result. Volatile contaminations of industrial cleaners such as tens ides desorbed from the surface during gas nitriding. Non-volatile contamination and reaction layers such as phosphates and silicates are stable and hindered gas nitriding during the entire process period. The nitriding hindrance of non-volatile contaminants was found to increase with increasing chromium and aluminium content of the steel and decreasing nitriding time.
Kurzfassung Seit einigen Jahren gehört die Niederdruckaufkohlung zum festen Bestandteil für den Prozess der Einsatzhärtung in der Getriebe- und Luftfahrtindustrie und in einigen Lohnbehandlungsbetrieben. Speziell in dem Bereich der Einsatzhärtung von Bauteilen für die Einspritzung von z.B. Kraftstoffen ist die Gasaufkohlung in zunehmendem Maße von der Niederdruckaufkohlung, vor allen Dingen wegen der Randoxidationsfreiheit, der gleichmäßigen Aufkohlung komplexer Geometrien und der Möglichkeit der Hochdruckgasabschreckung, die zusätzlich Vorteile hinsichtlich der trockenen und verzugsarmen Abschrecktechnik, der Umweltkonformität sowie der Wirtschaftlichkeit besitzt, abgelöst worden [1–8]. In einigen Anwendungsfällen in der industriellen Praxis erfordern die Einsatzbedingungen hohe Einsatzhärtungstiefen bis zu einigen Millimetern. Die Vakuumtechnik ermöglicht in einfacher Weise die Durchführung dieser Wärmebehandlungen bei Temperaturen im Bereich von 1000 °C bis 1050 °C. So können die Zyklusdauern, die bei konventionellen Temperaturen benötigt werden, drastisch reduziert und, wenn nötig, auch sehr hohe Randkohlenstoffwerte erreicht werden. In diesem Beitrag werden die Übertragungseigenschaften des Aufkohlungsgases Acetylen bei diesen hohen Temperaturen vorgestellt und an ausgewählten Beispielen aus der industriellen Praxis die erreichten Einsparpotenziale herausgearbeitet.
Steel manufacturing affects not only the shape of a workpiece but also its surface structure and composition. Cooling lubricants, hydraulic oils, and machine greases form stable adsorption or reaction layers at the elevated temperatures and pressures which occur during the manufacturing process. Adsorption or reaction layers can work as barrier layers and inhibit surface modification reactions, as in gas nitriding. Surface layers containing oxygen, sulphur, and phosphorus have been prepared on different alloy surfaces. The aim of the investigation was to explore possible mechanisms of reaction inhibition and develop a more reproducible nitriding process in terms of reaction temperature and furnace residence time. In this study, samples were nitrided at extremely short residence times (1 < t < 20 min) to detect the effects of reaction and barrier layers on nitrogen acceptance during the initial stages of the nitriding process. Surprisingly, for all alloys examined, the technically clean reference samples never showed the highest nitrogen concentrations. On the other hand, nitride layer growth rates were increased when surfaces were covered with a thin oxide or phosphate layer before nitriding.
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